Currently, various proposals are being presented and discussed for the 802.11n extension to the 802.11 WLAN standard, which will allow for higher throughput WLAN devices. These proposals come from various wireless consortiums that include EWC, the Joint Proposal and WWiSE. The following describes aspects of these proposals relevant to the present invention.
FIG. 1 shows a Clear to Send (CTS) frame as a MAC control frame as defined in the 802.11 standard. The receiver address (RA) of the CTS frame is copied from the transmitter address (TA) field of the immediately previous Request to Send (RTS) frame to which the CTS is a response. The duration value is the value obtained from the Duration field of the immediately previous RTS frame, minus the time, required to transmit the CTS frame and its short inter-frame spacing (SIFS) interval. If the calculated duration includes a fractional microsecond, that value is rounded up to the next higher integer.
The CTS frame need not always follow a RTS frame as described in the 802.11e standard (section 7.2.1.2). It can be the first frame in an exchange and used for setting the Network Allocation Vector (NAV) for MAC level protection for the transmission to follow. When the CTS frame is sent as the first frame by the initiating station of an exchange, the CTS may be addressed to itself and is referred to as CTS-to-self.
FIG. 2 shows a Contention Free End (CF-End) frame, which is a MAC control frame that may be sent by the AP as a broadcast frame to reset the NAVs of all stations in the system and is described in the 802.11 standard. A station receiving a CF-End frame with the basic service set ID (BSSID) of the BSS, to which the station is associated, will reset its NAV value to 0. This resets any medium protection/reservation currently existing. The Duration field is set to 0. As shown in FIG. 2, the BSSID is the address of the STA contained in the AP. The RA is the broadcast group address. The FCS is the frame check sequence.
In 802.11n, proposals were made to implement support for Extended Range using a different physical layer (PHY) modulation scheme than that used for Normal Range, essentially creating two modes of operation. Extended Range STAs transmit and receive using Space Time Block Code (STBC) PHY modulation, whereas Normal range STAs transmit and receive using a non-STBC PHY modulation. In a Joint Proposal contribution to 802.11n, an approach is described for an AP to support a network of STAs operating in a dual mode, where the two modes are Extended Range and Normal Range. Secondary beacon and Dual CTS method together are used to support Extended Range in addition to Normal Range, A secondary beacon is transmitted with a secondary beacon bit set in the beacon to let stations know that the target beacon transmission time (TBTT) for this beacon has an offset. In the Dual CTS protection, stations start a TXOP with an RTS directed at the AP, and the AP responds with a first and second CTS separated by a point control function inter-frame spacing (PIFS). When dual CTS protection enabled, the AP should protect STBC TXOPs with a non-STBC CTS and non-STBC TXOPs with an STBC CTS. The protection frames shall set a NAV for the entire TXOP. STBC control frames shall be used in response to STBC frames if the Dual CTS protection bit is set. Non-STBC control frames shall be used otherwise. PIFS is used as the interval to separate the dual CTS for non-STBC RTS.
FIG. 3 shows a diagram from the WWiSE proposal presentation document on the self-managed Extended Range protection. Examples of signaling for dual mode protection of normal range (NR) and extended range (ER) stations are shown. Signal sequences 301-305 relate to enhanced distributed coordination function (DCF) channel access (EDCA) and signal sequence 306 relates to a HCF controlled channel access (HCCA) format. The AP protects TXOP for the NR STA and ER STA using signal sequences 301-302 respectively. The ER STA protects its TXOP in signal sequence 303. A signal sequence for an 11n NR STA is represented by signal sequence 304, and one for a legacy NR STA is represented by signal sequence 305. In signal sequence 306, the AP protects a TXOP for the STA using HCCA format. As shown, the AP sends either a CTS in response to an RTS from a particular stations and in the mode as used by the station that sent the RTS, or a CTS-to-self signal in the mode other than that of the RTS-sending station.
FIG. 4 shows a new HT information element according to the WWiSE proposed Extended Range. The AP signals new HT information elements in management frames such as beacon, probe response etc. to manage the BSS (for example to support Extended Range). The new HT information elements may also be present in all beacons and probe responses transmitted by a station in IBSS mode. The HT information elements contain fields such as Secondary beacon, dual STBC/CTS protection, etc. as shown in FIG. 4. According to the Joint Proposal, the length is not fixed and the size depends on the number of fields that are included. The fields shall be in the order as shown in FIG. 4, with any new fields appearing at the end of the existing fields. Any fields unknown to the STA shall be ignored.
According to the Joint Proposal specification, and the EWC specification, following are some definitions related to Power Save Multi-Poll (PSMP) feature. A Power Save Multi-Poll (PSMP) is a MAC frame that provides time schedule to be used by the PSMP transmitter and PSMP receivers. The time schedule begins immediately subsequent to the transmission of the PSMP frame. A downlink transmission (DLT) is a period of time described by a PSMP frame, which is intended to be used for the reception of frames by PSMP receivers. An uplink transmission (ULT) is a period of time described by a PSMP frame, which is intended to be used for the transmission of frames by a PSMP receiver.
FIGS. 5 and 6 show PSMP information element formats according to the EWC MAC specification. FIG. 5 shows a PSMP parameter set format in which the PSMP is of type/subtype Management Action Frame and broadcast address type. The PSMP parameter set is used to describe the DLT and ULT which immediately follows the PSMP frame. FIG. 6 shows the STA Info information element format details such as Traffic (flow) ID, STA ID, DLT offset and duration, ULT offset and duration.
FIG. 7 shows the PSMP sequence consisting of a DLT phase followed by a ULT phase. Multi TID Block ACK (MTBA) is used to send Block ACK for multiple TID flows.
A need exists to extend a dual mode protection to supporting multiple mode operation. The current art is not robust and efficient in medium usage because it does not provide a mechanism to recover any unused transmission opportunity (TXOP) duration protected by the dual CTS transmission. Under the current art scheme, if the STA runs out of data to transmit during the protected TXOP, the medium is wasted for the remainder of the TXOP. A need exists to provide MAC signaling to relinquish the remaining unused TXOP to the system.
A need also exists for the PSMP sequence to operate in a multiple mode system in a bandwidth efficient manner. The 802.11n specification contains inconsistencies with respect to allowing only ACK/MTBA in ULT and no data for unscheduled PSMP. Also, there is no guidance for truncation of TXOP under dual CTS protection for STAs that are not able to interpret the CF-End frame.